In Pokemon Sword and Shield, Gigantamax forms were introduced. While most of these just allow the Pokemon to use a more powerful version of a move they already know for 3 turns, a few Pokemon have moves that only they can know. But before I get to those, let me explain what each type of move does. It is worth noting the power of most of these moves is dependent on the base power of the non-max version.
If you use a status move, it will become Max Guard. This protects you from any damage.
Using a Normal (Strike), Bug (Flutterby), Ghost (Phantasm), Dragon (Wyrmwind), or Dark (Darkness) move will decrease the speed, special attack, defense, attack, or special defense respectively of the opponent and its ally by one stage.
Using a Poison (Ooze), Fighting (Knuckle), Flying (Airstream), Ground (Quake), or Steel (Steelspike) move will increase the special attack, attack, speed, special defense, or defense respectively of the user and its ally by one stage.
Using a Fire (Flare), Ice (Hailstorm), Rock (Rockfall), or Water (Geyser) move will cause sunlight, hail, sandstorm, or rain respectively.
Using a Grass (Overgrowth), Psychic (Mindstorm), Electric (Lightning), or Fairy (Starfall) move will cause grassy, psychic, electric, or misty terrain respectively.
All of the following moves deal damage.
Venusaur’s max move (Grass) is Vine Lash, which deals damage to non-Grass types for four turns, equal to 1/6th of their maximum HP.
Charizard’s max move (Fire) is Wildfire, which deals damage to non-Fire types for four turns, equal to 1/6th of their maximum HP.
Blastoise’s max move (Water) is Cannonade, which deals damage to non-Water types for four turns, equal to 1/6th of their maximum HP.
Butterfree’s max move (Bug) is Befuddle, which causes sleep, paralysis, or poison.
Pikachu’s max move (Electric) is Volt Crash, which paralyzes opponents (assuming they can be).
Meowth’s max move (Normal) is Gold Rush, which confuses opponents (assuming they can be).
Machamp’s max move (Fighting) is Chi Strike, which raises Machamp’s critical hit rate.
Gengar’s max move (Ghost) is Terror, which prevents opponents from fleeing (assuming they can be prevented from doing so).
Kingler’s max move (Water) is Foam Burst, which lowers the speed of opponents by two stages.
Lapras’s max move (Ice) is Resonance, which sets up Aurora Veil for 5 turns even if Hail is not active.
Eevee’s max move (Normal) is Cuddle, which infatuates opponents of the opposite gender (assuming they can be).
Snorlax’s max move (Normal) is Replenish. If it was holding a berry and it lost it, it has a 50% chance to recover it.
Garbodor’s max move (Poison) is Malodor, which poisons all opponents (assuming they can be).
Melmetal’s max move (Steel) is Meltdown, which subjects the opponent to Torment (i.e. they now cannot use the same move twice in a row).
Rillaboom’s, Cinderace’s, and Inteleon’s max move are called Drum Solo, Fireball, and Hydrosnipe, respectively. They are Grass, Fire, and Water type, respectively. They are 160 base power regardless, and they all ignore the opponent’s abilities (i.e. Drum Solo ignores Sap Sipper, Fireball ignores Flash Fire, and Hydrosnipe ignores Storm Drain, Water Absorb, and Dry Skin).
Corviknight’s max move (Flying) is Wind Rage, which removes any screens, terrains, or entry hazards.
Orbeetle’s max move (Psychic) is Gravitas, which intensifies gravity for five turns.
Drednaw’s max move is Stonesurge, which sets up Stealth Rock on the opponent’s side of the field.
Coalossal’s max move (Rock) is Volcalith, which deals damage to non-Rock types for four turns, equal to 1/6th of their maximum HP.
Flapple’s max move (Grass) is Tartness, which lowers the evasiveness of all opponents.
Appletun’s max move (Grass) is Sweetness, which heals Appletun and its allies of any status conditions.
Sandaconda’s max move (Ground) is Sandblast, which sets up Sand Tomb on the opponent for four to five turns. This will continue even if Sandaconda is knocked out or switched out before four turns pass.
Toxtricity’s max move is Stun Shock, which either paralyzes or poisons individual targets.
Centiskorch’s max move (Fire) is Centiferno, which sets up Fire Spin on the opponent for four to five turns. This will continue even if Centiskorch is knocked out or switched out before four turns pass.
Hatterene’s max move (Fairy) is Smite, which confuses opponents (assuming they can be).
Grimmsnarl’s max move (Dark) is Snooze, which has a 50% chance to cause the opponent to become drowsy (assuming that can be inflicted).
Alcremie’s max move (Fairy) is Finale, which restores Alcremie’s HP by 1/6th of its max health.
Copperajah’s max move (Steel) is Steelsurge, which sets up a unique entry hazard that deals damage based on how weak it is to Steel. For example, it would deal around 3.125% damage to a Lanturn, 6.25% to a Blastoise, 12.5% to a Greedent, 25% to an Alcremie, and 50% to an Aurorus.
Duraludon’s max move (Dragon) is Depletion, which reduces the PP of the opponent’s last used move by 2.
Urshifu’s max move is dependent on its form. Single Strike’s is Dark and called One Blow, while Rapid’s Strike is Water and called Rapid Flow. Both of them ignore protection moves, including Max Guard.